Under the Hood: Fixing conductive stripes in defogger system

Q: My rear window has a strip of glass that doesn’t defrost like the rest of the window. It bugs me because it’s right in my view behind the car. What can be done to fix this? -Irlinda M.

A: It sounds like one of the slender printed conductive stripes on your rear glass has been scratched, severing its electrical conductivity. It’s possible, using a Permatex 09117 Complete Rear Window Defogger Repair Kit or similar, to repair a broken spot in these stripes. If careful visual inspection doesn’t indicate the point of breakage, an inexpensive multimeter can be used to identify the exact location.

With the window defogger turned on and the multimeter set to DC volts 20, connect the black meter lead to bare metal on the body, such as a door latch striker. Then gently touch the red lead tip to each stripe, one at a time, near the center of the glass. A functioning stripe will indicate roughly half of the vehicle’s system voltage – about 6V. This is because the stripes are long resistors and drop voltage along their length, from the source side (12V) to the ground side (0V). If you find a stripe that reads either 12ish or zero near the center, you’ve found the bad one!

Next, move gently left or right, probing that stripe, until the voltage abruptly changes. Search for the exact spot where you find 12ish in one place, and zero right next to it – this is the break! A full voltage drop in one place occurs because current isn’t flowing through that circuit. Apply a piece of tape directly above or below to mark the spot and break out the kit.

The kit consists of instructions, a cleaning pad, stencil, a slender brush and a bottle of conductive paint to apply over the scratched stripe. Also included are materials to re-bond a broken wire tab, if needed. You may find firmly applying your own pieces of tape may more accurately match the width of the stripe, and the paint color may not match exactly. Also, if sufficient thickness isn’t built up (three coats are recommended) the repair spot will heat a bit more than the rest of the stripe, defrosting a little sooner than elsewhere. If applied carefully the repair will work and is only slightly noticeable.

Q: How often should brake fluid be replaced? It’s not listed in my maintenance schedule. My oil change guys are trying to sell this! -Zach T.

A: This is an interesting topic, as some manufacturers are all over this, requiring fluid replacement every two or three years, or by mileage such as 30,000 or 45,000, while others imply it’s never needed! Brake fluid lives in a fairly sealed environment, but by its nature absorbs moisture, and its fluid additives break down over time. Moisture buildup lowers the boiling point of the fluid, leading to possible high temperature braking loss, and coupled with additive losses, encourages corrosion and gunking of some potentially expensive ABS parts.
Brake fluid typically darkens in color with age, from either clear or honey color to a deeper brown, but this isn’t a reliable indicator of true condition. Test strips are available to check both moisture and copper content; the latter is an excellent indicator of fluid additive depletion and system corrosion. As you found, quick lube shops may also check condition for you in the hopes of selling a flush. I flush/replace fluid on my vehicles every three or four years even though it’s not listed in the book. ABS systems can complicate fluid replacement, often times requiring the use of a scan tool to shuttle various valves in the right way to pass fluid through.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at bradbergholdtgmail.com; he cannot make personal replies.